In aqueous ink indirect printing, an aqueous ink is jetted on to an intermediate imaging surface, which can be in the form of a blanket. The ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality it is desirable that the ink drops jetted onto the blanket spread and become well-coalesced prior to drying. Otherwise, the ink images appear grainy and have deletions. Lack of spreading can also cause missing or failed inkjets in the printheads to produce streaks in the ink image. Spreading of aqueous ink is facilitated by materials having a high energy surface.
However, in order to facilitate transfer of the ink image from the blanket to the media substrate after the ink is dried on the intermediate imaging surface, a blanket having a surface with a relatively low surface energy is preferred. Rather than providing the desired spreading of ink, low surface energy materials tend to promote “beading” of individual ink drops on the image receiving surface.
Thus, an optimum blanket for an indirect image transfer process must tackle both the challenges of wet image quality, including desired spreading and coalescing of the wet ink; and the image transfer of the dried ink. The first challenge—wet image quality—prefers a high surface energy blanket that causes the aqueous ink to spread and wet the surface. The second challenge—image transfer—prefers a low surface energy blanket so that the ink, once partially dried, has minimal attraction to the blanket surface and can be transferred to the media substrate.
Various approaches have been investigated to provide a solution that balances the above challenges. These approaches include blanket material selection, ink design and auxiliary fluid methods. With respect to material selection, materials that are known to provide optimum release properties include the classes of silicone, fluorosilicone, a fluoropolymer, such as TEFLON or VITON, and certain hybrid materials. These materials have low surface energy, but provide poor wetting. Alternatively, polyurethane and polyimide have been used to improve wetting, but at the cost of ink release properties. Tuning ink compositions to address these challenges has proven to be very difficult since the primary performance attribute of the ink is the performance in the print head. For instance, if the ink surface tension is too high it will not jet properly and it if is too low it will drool out of the face plate of the print head.
Additional attempts at solving the above challenges have included applying a sacrificial wetting enhancement composition to form a sacrificial coating (also known as “skin”) onto the blanket to improve wetting and spread of ink while maintaining transfer capabilities. Much focus has been placed on developing formulations for the sacrificial wetting enhancement coating to improve shelf life and mechanical properties thereof. Despite the progress in developing new sacrificial wetting enhancement coating formulations, the conventional method of applying the skin formulation is via a surface maintenance unit that utilizes a coating application, such as a donor roller. The donor roller can be, for example, an anilox roller or elastomeric roller made of a material, such as rubber, and is partially submerged in a reservoir that holds a sacrificial coating composition. The donor rotates in response to movement of an image receiving surface and draws liquid sacrificial coating composition from the reservoir and deposits a layer of the composition on the image receiving surface. Unfortunately, such a nonselective coating method for applying the sacrificial coating composition results in flooding the whole print medium, for example, a whole sheet of paper. This results in waste as the same amount of sacrificial coating composition is applied for both low and high coverage prints, even though only the imaging area requires the skin. Accordingly, excess skin can cause many issues for cleaning during each imaging cycle, and the waste results in higher cost.
Identifying and developing new methods for applying such sacrificial coating compositions to overcome and embodying such methods in new printing apparatuses would be considered a welcome advance in the art.